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117Polymers from Renewable Resources, Vol. 3, No. 3, 2012

Experimental Study on Mechanical and Thermal Properties of Epoxy Composites Filled with Agricultural Residue

©Smithers Rapra Technology, 2012


G.U. Raju1* and S. Kumarappa2

1Department of Mechanical Engineering, B. V. B. College of Engineering and Technology, Hubli-580 031, Karnataka, India 2Bapuji Institute of Engineering and Technology, Davangere-577 004, Karnataka, India

Received: 17 February 2012, Accepted: 31 May 2012

SuMMARy

In the last twenty years, the use of lignocellulosic fibers as filler to produce polymer composites has increased progressively. A lot of research work has been performed all over the world on the use of natural fibers as reinforcing material for the preparation of composites. The objective of the present work is to prepare a polymer based composite material using agricultural waste as reinforcing material and characterize some mechanical and thermal properties. In this investigation, groundnut shell particles were chemically modified and used with epoxy to form novel bio-based composites. Composite boards were fabricated with the different weight percentages of groundnut shell particles and epoxy resin. The maximum strength was observed for the sample A1 having 50 wt% filler content and 0.5 mm particle size. However, the sample A3 with 85 wt% filler content and 1mm particle size has maximum MOE. It was observed that thermal conductivity of composite specimens range from 0.07638 to 0.3487 W/m-K and linear thermal expansion varies from 0.725 x 10-6 to 1.296 x 10-6/°C. The results of present study have showed that groundnut shell particles could be used successfully to develop a beneficial composite that would be a substitute for wood-based panels in many applications.

Keywords: Agricultural waste, Groundnut shell particles, Epoxy resin, Mechanical properties and Thermal properties

*Correspondingauthor,E-mail:[email protected],[email protected](G.U.Raju)

118 Polymers from Renewable Resources, Vol. 3, No. 3, 2012

G.U. Raju and S. Kumarappa

INTRODuCTION

In the recent years, cellulose based plant fibers extracted from biomass attracted attention of researchers and are experiencing increased demand as reinforcing materials for polymer matrices. This is because natural fibers have the potential of serving as alternative for artificial fiber composite. The extensive use of lignocellulosic fibers and their composites is for the reason that they provide several advantages such as low densities, low cost, nonabrasive nature, high filler loading, low energy consumption, high specific properties, biodegradability and safe working environment. This excellent property profile has made natural fibers more attractive for different industrial applications. In recent years, major industries such as automotive, construction and packaging industries have shown immense interest in developing the new biocomposite materials. Researchers are currently engaged in developing new and alternate products to synthetic fiber reinforced composites.

Recent advances in the use of natural fibers such as flax, cellulose, jute, hemp, straw, sisal, kenaf, coir and bamboo in composites have been reviewed by several authors [1-6]. The extensive studies on the preparation and properties of thermoset and thermoplastic composites filled with natural fibers such as, bagasse [7, 8], pineapple [9], rice husk [10, 11], kenaf [2, 13], jute [14-16] and sisal [17-18] have also been carried out.Use of these agricultural crop residues couldopennewmarketsandimproveruralagriculturebasedeconomy.RezaurRahman [19] studied on jute fiber reinforced polypropylene composites. Four levels of fiber loading (20, 25, 30, and 35 wt%) were used for composite manufacturing and SEM analysis and mechanical tests were conducted. It was observed that post-treated jute fiber reinforced specimens yielded bettermechanicalpropertiescomparedtotheoxidizedandrawonesand30% filler loaded composites had the optimum set of mechanical properties. Hulya Kalaycıoglu et al. [20] investigated the usage of kenaf stalks as a raw material for particleboard manufacturing. The study suggests that kenaf is superior of other agricultural residues in terms of its dimensional stability and can be used to manufacture particleboard. Experimental results show that both physical and mechanical properties of the panels are comparable to boards made from other agricultural residues. Chittaranjan Deo [21]investigated the moisture absorption behavior and its effect on mechanical propertiesofLantanacamarafiberreinforcedepoxycomposite.Compositesamples reinforced with different wt% of fibers were prepared. It was observed that, with increase in fiber content up to 30%, tensile and flexural strength was increased. Moisture absorption tests were carried out in three differentenvironmentalconditionssuchassteam,salinewaterandsub-zerotemperature. The moisture absorption of the composite was higher for steam environmentthanthatforsalinewaterandsub-zerotemperatureenvironments



and the tensile and flexural strength of composite were found to decrease with moistureabsorption.Bledzkietal.[22]investigatedtheeffectofdifferentfiberloading and coupling agent on the microstructure and mechanical properties of abaca fiber reinforced polypropylene composites. Optimal fiber loading was found to be 40 wt% and the adhesion between abaca fiber and PP matrix has significantly improved due to addition of MAH-PP coupling agent. The results are compared with jute-PP and flax-PP composites. It was observed that Jute fiber-PP composites showed better tensile properties and flexural modulusthanabaca-PPandflax-PPcomposites.Butabaca-PPcompositesshowed better flexural strength than jute-PP and flax-PP composites. Shih [23] studied on the composites reinforced with fibers obtained from water bamboo husks that were chemically modified by coupling agents. In addition, the powders obtained from water bamboo husks were also used, but without chemical modification. Epoxy resin was used as the matrix material to form novel composites. Morphologies, mechanical properties and heat resistance of these water bamboo husk reinforced composites were investigated. The morphology analysis reveals that the fibers modified by coupling agents exhibited better compatibility with the polymer matrices than the untreated fibers. Moreover, the thermal resistance was improved as the plant fibers and powders were individually incorporated to the polymers. The increments of char yields of epoxy were about 13.5–52.8% with the addition of 10% fiber or powder. It was also found that the glass transition temperature of epoxy was increasedabout8–18°C. Inaddition, themechanicalpropertieswerealso enhanced due to the addition of treated fibers and untreated powders. The increments of storage moduli of epoxy were about 16.4 and 36.1% with the addition of 10% coupling agent treated fibers and untreated powders, respectively.

Maries Idicula et al. [24] have investigated thermal conductivity, diffusivity and specific heat of polyester/ natural fiber (banana/sisal) composites as function of filler concentration and for several fibre surface treatments. The results show that chemical treatment of the fibres reduces the composite thermal contact resistance.Hybridizationofnaturalfiberwithglassallowsasignificantlybetterheat transport ability of the composite. It was observed that the banana/sisal fiber-polyester composites with 20 and 40 volume percentage of fibers have thermal conductivity in the range 0.153-0.140 W m-1K-1 and specific heat in the range 1199-1246 J kg-1K-1 respectively. The composites prepared with chemically treated fibers showed that the thermal conductivity of alkali treated fiber composite was 43% higher than the untreated fiber composite andthevariationofspecificheatwasnotsosignificant.BehzadandSaininvestigated thermal conductivity for hemp fiber reinforced composites [25]. The transverse and in-plane thermal conductivities for oriented and randomly oriented composites for different volume fractions of fiber were investigated.



Experimental results show that the orientation of fibers has a significant effect on the thermal conductivity of composites. Agarwal et al. [26] investigated the variation of thermal conductivity and thermal diffusivity of banana-fiber reinforced polyester composites with the addition of glass fiber and observed that the thermal conductivity of composites decreases with increased percentage of glass fiber when compared to composite of pure banana fiber. Xue Li et al. [27] carried out experimental studies on flax fiber–HDPE biocompositesinthetemperaturerangeof170–200°Ctofindoutthermalproperties such as thermal diffusivity, thermal conductivity, and specific heat. The fiber contents in biocomposites were 10%, 20%, and 30% by mass. It was found from experiment that the thermal conductivity, thermal diffusivity, and specific heat decreased with increasing fiber content, but thermal conductivity and thermal diffusivity did not change significantly with temperature in the range studied. The specific heat of the biocomposites increased gradually with temperature. The thermal conductivity and thermal diffusivity of oil-palm-fiber-reinforced untreated and differently treated composites using transient plane source technique at room temperature was studied by Agrawal et al. [28]. The study showed that all the saline and alkali treatments of the fibers increased the thermal conductivity and thermal diffusivity of the composites in comparison with the acetylated composite. The saline treated fiber has higher polarity because of the formation of silonal group on the surface that results the higher thermal conductivity of the saline treated composites. The alkalization treatment removes impurities and increases the fiber surfaceadhesion characteristic with the resin and contributes to a higher thermal conductivity. The acetylation slightly increased the polarity of the fiber and, hence, the thermal conductivity of the composite increased marginally. Alsina et al. [29] investigated the thermal properties of jute-cotton, sisal-cotton and ramie-cotton hybrid fabric reinforced unsaturated polyester composites. The results showed that sisal-cotton hybrid polyester composites have thermal conductivity 0.213-0.25 W/m-k and specific heat of 1.065-1.236 J/cm3°Cfor0.69 volume fractions of sisal. Jute-cotton hybrid polyester composites have thermal conductivity 0.10-0.237 W/m-k and specific heat of 0.869-1.017 J/cm3 °C for 0.64 volume fractions of jute. Ramie-cotton hybrid polyestercomposites have thermal conductivity 0.19-0.22 W/m-k and specific heat of 0.839-0.894 J/cm3°Cfor0.77volumefractionsofRamie.

Despite some unsolved problems regarding poor compatibility between natural fibers and polymer matrices, these composites found many applications in automotive sector, packaging, building and construction. A major deficiency in the natural fiber-polymer composites is the poor bonding between the hydrophilic natural fiber and hydrophobic polymer. It is indispensable to impart hydropobicity to natural fibers by suitable techniques such as chemical treatment, acetylation and graft co-polymerization. These treatments are



used to develop composites with better mechanical and thermal properties. The chemical treatment allows a better contact between the fiber-matrix and reduces the thermal contact resistance considerably [24]. It has been reported that NaOH chemical treatment of fiber allows significant increase of thermal andmechanicalpropertiesofcomposites[30-32].Bythedissolutionofligninin alkali some pores are formed on the fiber surface that improves the contact area between the fiber and the matrix.

Until now, groundnut shell was not used as reinforcement in polymer resin for the development of composite material. As per authors’ knowledge, no work has been reported in the literature on properties of groundnut shell particles reinforced polymer composite materials. Hence, an attempt has been made in this paper to develop composite material employing groundnut shell particles as filler in epoxy polymer matrix and investigated some mechanical and thermal properties.

MATERIALS AND METHOD

Groundnut Shell

Groundnut botanically known as Arachis hypogeae belongs to Leguminosae family. It is the fourth largest oilseed produced in world and India is the second largestproducerofgroundnutafterChina.InIndia,groundnutisthelargestoilseed in terms of production and accounted for about 7.5 million tonnes during 2009-10. A complete seed of groundnut is called as pod and outer layer of groundnut is called shell. Groundnut shell chemical composition is compared with the selected natural fibers and is presented in the Table 1. The hemicelluloses content of the groundnut shell is found to be 18.7%, cellulose 35.7%, lignin 30.2% and ash content 5.9%. Lignin is often called the cementing agent that binds individual fiber cells together and hemicellulose content in the groundnut shell is liable for moisture absorption. The lignin content of groundnut shell fiber is much greater than that of banana, bagasse, rice husk, jute, hemp, kenaf and sisal fibers. The hemi cellulose content of groundnut shell is less than wood, banana, baggase, rice husk and kenaf fibers. Pre-treated groundnut shell is used in this study to modify the surface properties toensureinterfacialinteractionsbetweentheparticlesandtheresin.Cleanand dried groundnut shells were first washed with water to remove the dirt and impurities. Then washed shells were chemically treated with 10% NaOH solution for 2 hours and then washed with distilled water until all NaOH gets eliminated. Subsequently, the shells were solar dried and ground. Then the particlesweresievedthrough0.5,1,2and3mmBSsievestogetdifferent



size groundnut shell particles. These particles are used as reinforcementmaterial in epoxy polymer matrix.

Table 1. Chemical composition of some natural resources

Species Cellulose(wt%)

Hemicellulose (wt%)

Lignin (wt%)

Ash (wt%)

Reference

Banana 63-64 19 5 - [24]

Pineapple 81 - 12.7 - [24]

Coir 32-43 0.15-0.25 40-45 - [33]

Sisal 63-64 12 10-14 - [33]

Jute 61-71.5 12-20.4 11.8-13 2 [34]

Kenaf 31-39 21.5 15-19 - [34]

Hemp 70.2-74.4 17.9-22.4 3.7-5.7 - [34]

Bagasse 40-46 24.5-29 12.5-20 1.5-2.4 [35]

Groundnut shell 35.7 18.7 30.2 5.9 [36]

Rice husk 31.3 24.3 14.3 23.5 [36]

Polymer Resins

In the present study epoxy resin was used as matrix materials. Epoxy resin exhibit extremely high resistance alkali, good acids and solvent resistance and has good electrical properties over a range of frequencies and temperature. The cured epoxy systems generally exhibit good dimensional stability, thermal stability and exhibit resistance to most fungi. They are self-excellent moisture barriers exhibiting low water absorption and moisture transmission. Epoxy used in the preparation of composite is of the category LY554 along with the hardener HY951 and is cured at room temperature. Resin to hardener ratio used was 10:1. Hardener is typically amine or mixture of amines.

Mechanical and Thermal Properties

Preparation of Composite Boards

A mould with suitable dimensions was used to prepare the groundnut shell particles reinforced epoxy composite specimens. Initially a layer of wax (releasing agent) was applied to the mould so that the specimen can be easily taken out of the mould. Proper measured quantity of groundnut shell particles and epoxy resin were taken in a plastic container and stirred thoroughly to get homogeneous mixture. After adding the hardener, the mixture was again



stirred for 10 minutes and thoroughly mixed mixture was placed in the mould and compressed uniformly. This set up was kept for 24 hours of time and after curing composite board was taken out from the mould. Three compositions of groundnut shell particles and epoxy resin with 50, 65 and 80 weight percentagesofreinforcingparticleswithdifferentparticlesizesof0.5,1,2and 3 mm were prepared. Groundnut shell particles reinforced epoxy polymer composite(GPEC)boardsweredesignatedwiththeparticlesize.SeriesAindicatesparticlesize0.5mm,seriesBis1mm,seriesCis2mmandseriesD is 3 mm. Numeral 1, 2 and 3 indicates weight percentage of groundnut shell particles as 50, 65 and 80 respectively. Specimen A-1 indicates 0.5 mm particlesizewith50wt%offiller,specimenB-2indicates1mmparticlesizewith65wt%offiller,C-3indicates2mmparticlesizewith80wt%offiller,D1indicates3mmparticlesizewith50wt%offillerandsoon.Thedensityvalues of the specimens prepared were between 676.9-898.1 kg/m3 and are of medium density boards (600-900 kg/m3)asindicatedbyBISspecifications.

The specimens for flexural, tension, impact, and moisture absorption tests havebeencutfromtheGPECboardsasperthedimensionsspecifiedinASTMstandards. The standards used are ASTM D638 for tension, ASTM D790 for flexural, ASTM D256 for impact, and IS: 2380(PART XVI) for moisture absorption.

Bending Test

The test method determines the flexural properties of groundnut shell particles reinforcedepoxycomposites(GPEC)inaccordancewithASTMD790.Flexuraltests were performed using Universal Testing Machine at a constant rate of 2 mm/min. Five test specimens were cut to 191 mm x 13 mm and 10 mm depth. The span length for loading the specimen was 150 mm. Flexural strength and flexural modulus were calculated using the Equation (1) and Equation (2).

MOR (Flexural strength) = 3PL

2b h2 MPa (1)

MOE (Flexural modulus) = m L3

4b h3 MPa (2)

Where P = maximum load applied on test specimen (N) L = support span (mm)

b = width of specimen tested (mm), and

h = thickness of specimen tested (mm)

m = slope of tangent to the initial straight line portion of load deflection curve (N/mm)



Tensile Test

The test method covers the determination of the tensile properties of groundnut shellparticlesreinforcedepoxycomposite(GPEC)boardsinaccordancewithASTMD638testprocedure.Acomputerizeduniversaltestingmachinewasused to conduct the tension test. The test specimen of 50 mm gauge length, 29 mm width and 10 mm thickness was placed between the holders of the universal testing machine and the constant rate of loading of 5 mm/min was applied. Stress, strain and Young’s modulus were calculated. The computed values of young modulus are based on the slope of the linear portion of the stress-strain curve. Five specimens for each sample were tested and tensile strength and tensile modulus was calculated using the Equation (3) and Equation (4).

Tensile strength (MPa) = P

b h (3)

Tensile modulus (MPa) = σ

ε (4)

Impact Test

ThetestmethoddeterminestheIzodimpactstrengthofGPECspecimensin accordance with ASTM D256. The Impact testing machine and V notched specimen was used to conduct the impact test. Specimen of 63.5 mm length, 10 mm width, 10 mm thickness with the depth of the notch 2.54 mm and notch angle of 45°

wasusedforthetesting.InIzodtestthespecimenwas

held as a cantilever beam (usually vertical) and was rigidly clamped with the centre line of the notch on the level of the top of the clamping surface. The pendulum was released to strike the specimen and energy absorbed by the specimen was noted down directly on the scale. Five samples in each composition were tested and the mean value was taken as the impact strength oftheGPECspecimens.Theimpactstrengthofthespecimenwascomputedusing the Equation (5).

Impact strength (KJ / m2 ) = J

A (5)

where J = Energy absorbed

A = Area of cross section of the specimen below the notch.



Moisture Absorption Test

ThetestmethoddetermineswaterabsorptionofGPECboardsinaccordancewith IS: 2380 (part 1)-1977. The specimens were prepared from a 10 mm-thick platewithsize50mmwideand75mmlong.Thespecimensweresubmergedhorizontallyunder25mmfreshcleanwatermaintainedatatemperatureof27±2°C.Thespecimenshallbeseparatedatleast15mmfromeachotherand from the bottom and side of the container. After 2 hours, the specimens were taken out and suspended to drain for 10 minutes, so as to remove the excess surface water and then weighed. The specimen was then submerged again and the above weighing procedure was repeated after 24 hours. The specimens were immersed in water for a period of 15 days. The moisture absorption of the composite was measured by the weight gain of the material at regular interval of 24 hours. The percent moisture was calculated using the Equation (6) as the ratio of increase in mass of the specimen to the initial mass.

%moisture absorption =

mf −m0

m0

x100

(6)

Where mo = Initial mass

mf = Final mass

Thermal Conductivity Test

A circular disc shaped GPEC specimens of size 130 mm diameter and10mmthickwerepreparedfordifferentparticlesizeandweightpercentageof reinforcement and are designated from A1 to D3 as described previously. The method used to measure the thermal conductivity of composites has beenadaptedfromatechniquepresentedinBehzadandSainwork[25].Theexperimental setup consists of heating element, connected to a conducting materialofsamesizeasthatofthespecimen.Threethermocoupleswereconnected to the specimen at different points. The entire setup was placed in a thermally insulated evacuated chamber in order to prevent loss of heat from specimen. The heat supplied was maintained constant until steady state is reached and then temperature at the thermocouples was noted using digital temperature indicator. The thermal conductivity was determined, measuring temperatures, heat supplied and area of the specimen by using a discrete approximation of Fourier’s law for one-dimensional heat conduction, given by:

Q =KA dT

dx (7)



where, Q: Heat dissipated through the plate; K: thermal conductivity of the composite plate; A: surface area of the specimen; dT: temperature difference (T1-T2); dx: thickness of the disc. The validation of the test was done through measuring the thermal conductivity of known material such as neat epoxy. It was found that the determined thermal conductivity value substantiate the standard value with the greatest accuracy.

Linear Thermal Expansion Test

Specimensofsize150x40x10mm3 were prepared for different particle sizeandweightpercentageof reinforcementandaredesignated fromA1to D3 as described previously for carrying out the linear thermal expansion test. The experimental setup consists of heater and conductive material such as aluminum plate. The specimen was kept over the plate and dial gauges were placed at the ends of the specimen at different points to measure the deflection. Thermocouples were placed in the specimen and are connected to the temperature-measuring instrument. The initial temperature of the specimen was recorded and then heated uniformly. For the increase in temperature, corresponding deflection of the specimen was recorded at equal intervals. ThecoefficientofthermalexpansionofGPECspecimenswasdeterminedby averaging five readings and using the following formula:

L = L0(1+αt) (8)

Where, L0 and L: initial and final length of the specimen at temperatures Ti and Tf; α: coefficient of thermal expansion; Ti and Tf: initial and intermediate temperatures of the specimen; t=(Tf -Ti) temperature difference.

RESuLTS AND DISCuSSION

The results obtained from the mechanical tests carried out are presented in Figures 1 to 5 and the mechanical properties in relationship with the filler content are also shown. As can be seen from Figures 1 and 2, it was observed that particle size and filler content are the influencingparameters on theproperties of the composite. Strength of the composite specimens decreased significantlywith increase inparticlesizeandfillercontent.BasedonENstandard 312-2 and 312-3, 11.5 N/mm2 and 1600N/mm2 are the minimum requirements for MOR and MOE of panels for general purpose and interior fitments,respectively[37-39].AlltheGPECboardsexcludingboardsofDseries satisfies the MOR requirements. Owing to the application of flexural load in three-point bending test, the upper and lower surfaces of the specimen



Figure 1. MOR of GPEC samples

Figure 2. MOE of GPEC samples



were subjected to compression and tension stress and axisymmetric plane is subjected to shear stress. The specimen fails when bending or shear stress reaches the critical value. From the results of the study, a linear behaviour of the composite specimens was observed for MOR with respect to filler loadingandparticlesize.Ahigherstrengthvaluewasobservedforthesmallerparticlesizeandlowerfillercontentowingtothewettabilityoftheparticlesand enhanced adhesion between the particles and matrix. The maximum MOR of the composite was found to be 22.0 MPa at 50 wt% filler loading and the values decreases with further filler loading. Similar results have been reported by several authors that the flexural strength decreased after 40 wt% of filler loading due to insufficient wetting of resin to the fillers [13, 40]. However, opposite trend was observed for MOE property of the composite specimens. MOE value increased with increase in filler loading and is due to improvement in stiffness of the composite specimens with the addition of reinforcement filler. It is for the reason that reinforcing particles have higher stiffness than the resin matrix. Similar results were also reported by other researchers[14,41].AlltheGPECboardshavinghigherfillercontentsof65and 80 wt% satisfies the MOE requirements for general purpose use. 1 mm particlesizewith80wt%offillerwasfoundtobehavingmaximumMOEvalueof 3074.4 MPa. From the results of the experiments shown in Figures 1 and 2,MORofGPECspecimensareintherangeof10.2-22.0MPa,indicatingmaximum for sample A1 and MOE are in the range of 1010.1 -3074.4 MPa withsampleB3havingmaximumMOE.

The variation of the tensile strength and Young’s modulus of the GPECspecimens at different filler loading are publicized in Figures 3 and 4 respectively. The strength of composite depends on some factors such as filler loading and the bonding between filler and matrix. From the results of the experiments,ithasbeenobservedthatGPECspecimensshowadecreasein tensile strength with an increase in filler content. This could be due to the reason that during tensile loading partially separated microspaces are created that obstructs stress propagation between the fiber and the matrix [43]. As the fiber loading increases, the degree of obstruction increases, which in turns decreases the strength of the specimens. Further, inadequate wettability of the particles due to higher filler loading resulted in poor filler-matrix adhesion and thus decreased tensile strength. Also, results evidenced that the tensile strength of the specimens increased with reducing particle sizes.Thismightbeduetothefactthatsmallerparticlesizeledtothelowervoid spaces in composites that arise at better bonding between particles and resin. The results observed in this study attributed that the fillers had created some reinforcing effect and had been responsible for the increase in tensile modulus. It was also observed by several authors [14, 32, 42] that, the Young’s modulus increased with an increase in filler loading. This observation



Figure 3. Tensile strength of GPEC samples

Figure 4. Young’s modulus of GPEC samples



may possibly lead to the conclusion that tensile modulus does not depend on the particle–matrix interface properties, but perhaps on the filler content. Increase in tensile modulus for higher filler loads are due to the particles with higher stiffness than the matrix material. Owing to this, overall stiffness of the composite specimens increased and thus tensile modulus enhanced. From the study, it was evident that the most significant effect of particles is the increase in modulus of the composite specimens. The tensile strength of GPECspecimensareintherangeof4.9–9.8MPaindicatingmaximumtensilestrength for sample A1 and the sample A3 has maximum Young’s modulus of 246.7 MPa. The tensile strength of all the boards showed higher values than theBIS-specifiedvalues.Also,thestrengthvaluesarereasonablyhigherthanthe composite made from areca natural fiber [33].

TheimpacttestresultsoftheGPECspecimensareshowninFigure 5. The composite samples exhibit increase in impact strength for smaller particle sizeofthefiller.Thisbehaviourindicatesimprovedfiller/matrixadhesionandwasduetolowervoidspacesincompositesbecauseofsmallersizeoftheparticles. Impact strength decreased for increase in weight percentage of filler content. Similar behaviour composites due to filler loading were also observed by other researchers [32, 42]. This may be attributed to the weak interfacial interaction between the filler and matrix materials for higher filler

Figure 5. Impact strength of GPEC samples



content.Also,impactstrengthdecreasedforincreaseinparticlesize.Thisbehaviourisbecauseofthesampleswithhigherparticlesizesalsohavehighersurface area, which could leave more surfaces, exposed for the case of poor bonding, thus resulting in decrease of strength. There is a steady decrease in impact strength with increase in filler content, indicating maximum value of 34.3 kJ/m2 for A1 specimen.

Moisture absorption values obtained from the tests are presented in Figure 6. It has been observed from the moisture absorption test that, quantity of moisture absorbed in the composite increases with time and later it happens to be constant. The percent amount of moisture absorbed in the composite for the first day is greater than in subsequent days. From the results, it is apparent that percentagemoistureabsorptionofGPECcompositesincreaseswithincreaseinfillercontentandparticlesize.Itisduetothefactthatincreaseinthemoisturecontent of the natural plant particle results in swelling, because the cell wall polymers of the material contain hydroxyl or other oxygen-containing groups that attract water through hydrogen bonding. The hemi-celluloses are the most hygroscopic. It is the moisture that swells the cell walls and causes the expansion of the material until the cell walls are saturated with water. This can obviously give rise to degradation as a result of attack by micro-organisms as wellasbulkinganddimensionalinstability.MoistureabsorptionvaluesofGPECcomposites vary from 23.12% to 38.56% for 15 days, as depicted in Figure 6. MoistureabsorptionintheseGPECsamplesisrelativelyminimumcomparedto

Figure 6. Moisture absorption of GPEC samples



other agro-based composite materials [38, 39]. Mehmet Akgul [38] used corn stalk agricultural residue mixed with oak wood fiber with urea formaldehyde resin to prepare the particle boards. In their study, water absorption values for 24 h were found to be 48-62% depending on the density 0.7 to 0.8 g cm-3 of theboards.Bhaduriin[39]intheirstudyusedkhimpplantstemwithphenolformaldehyde and urea formaldehyde resin to prepare the boards. The water absorption values for 24 h were 34-61% depending on the different percent ofresin.Thesampleswithsmallerparticlesizeabsorblessmoistureduetolower void space in composite arising at better bonding between particles and resin. A significantly low amount of moisture was held by the composites samplesof0.5mmparticlesize.Ingeneral,moistureabsorptionvaluesoftheGPECboardspreparedconfirmedtotheISspecifications.Moistureabsorptioncould be further reduced by adding water repellent chemicals such as paraffin during board production [44].

The scanning electron microscopy (SEM) was employed to study the morphologicalinvestigationsofGPECcompositesandshowninFigure 7. Through SEM study, the effect of filler loading and the bonding between the particles and polymer matrix in the composite board could be observed. The smallersizeoftheparticlesandlowfillerloadingresultedinimprovedwetting

Figure 7. (a) 50 wt% filler composite specimen with 0.5 mm particle size (b) 80 wt% filler with 0.5 mm particle size (c) 65 wt% filler composite specimen with 1 mm particle size

(a)

(b) (c)



of particles with resin and better filler–matrix adhesion. Thus the mechanical properties of the composite boards improved and also, moisture uptake of these boards is less. As the filler quantity increased, the capability of particles to impregnate into the resin decrease that induces the formation of voids in the composite boards and causes weak filler-matrix adhesion and thus poor mechanical properties at higher filler loading. This was observed in the SEM images of Figure 7a and b.Thesampleswithhigherparticlesizesalsohavehigher surface area, which could leave more surfaces exposed for the case of poor bonding at higher filler loading, thus resulting in decrease of strength and increased moisture absorption of the composite boards as seen from Figure 7c.

The thermal conductivity for all groundnut shell particle reinforced epoxy composite(GPEC)specimenswasobtainedbyaveragingfivemeasurementsat various positions of the prepared specimen and the average values are shown in Figure 8. It was found from the test that the composite materials have thermal conductivity in the range from 0.076 to 0.348 W/m-K. From the results of the experiment, it has been noticed that increase in the filler quantity decreases the thermal conductivity of the composite. This behavior of composite may be due to lower thermal conductivity of groundnut shell particle. Similar behaviour was also observed by several authors [24, 45]. Also

Figure 8. Thermal conductivity of GPEC specimens



withtheincreaseingrainsizeofthegroundnutshell,thethermalconductivityalso increases. It was observed in the study that the thermal conductivity of sodium hydroxide treated fiber composite was higher than the untreated fiber composite [24]. The NaOH treatment removes practically all non-cellulose componentsexceptwaxes.Bythedissolutionofligninbyalkali,someporesare formed on the fiber surface, which improves the contact area between the fiber and the matrix. The thermal conductivity of plain epoxy matrix is found to be 0.21 W/m-K. It has been found for the banana/sisal fiber-polyester composites [24] with 20 and 40-volume percentage of fibers have thermal conductivity in the range 0.153-0.140 W m-1K-1. Ramanaiah [45] investigated the thermal conductivity of broom grass/polyester composites with different fiber loadings and the values are in the range 0.196 -0.231W m-1K-1. From thestudy,ithasbeenfoundthatGPECcompositeboardswithhigherfillercontent have improved thermal conductivity value compared to broom grass/polyester composites and banana/sisal fiber-polyester composites.

The linear thermalexpansionforeachGPECspecimenswasobtainedbyaveraging five measurements at various positions of the prepared specimen and the average values are shown in Figure 9. From the test, it was found that the specimens have linear thermal expansion in the range from 0.72 x 10-6 to 1.29 x10-6/°C.Thecoefficientoflinearthermalexpansionofplainepoxymatrix is found to be 2.4 x 10-6/

oC.Fromthestudy, ithasbeenobserved

Figure 9. Linear thermal expansion of GPEC specimens



that increase in the filler quantity decreases the coefficient of linear thermal expansion. This behavior is perhaps due to lower thermal expansion of groundnutshellparticle.Itwasalsoobservedthatincreaseingrainsizeofthegroundnut shell particles increases the linear thermal expansion. Nevertheless, furtherincreaseinthesizeoftheparticlefrom2mm,thermalconductivityand thermal expansion values have not increased significantly. The behaviour oftheGPECcompositeforthermalexpansionissimilartothatofthermalconductivity property.

CONCLuSIONS

The principal objective of this experimental investigation was to prepare the compositebyutilizingagriculturalwasteandtoexaminethefeasibilityofusinghigh filler loading in polymer for the preparation of composite panel. The result shows that a useful composite with moderate strength could be successfully developed using groundnut shell particles as reinforcement in epoxy matrix. These composites are found to have good mechanical and thermal properties. However, groundnut shell particle reinforced epoxy composites (GPEC)exhibited low strength values compared to high-performance composites owing to high filler loading. Nevertheless, the mechanical properties are better than areca fiber–urea formaldehyde composites and coir-polyester composites [5].InadditionGPECsamplessatisfytheminimumrequirementasspecifiedby EN standard for MOR and MOE of panels for general purpose use. It was observed that the strength properties were better for 50% filler loading and smallerparticlesize.However,highmoduluswasobtainedfortheboardshaving higher filler loading. The highest tensile, bending, and impact strengths were observed for the panel A1 (having 50 wt% of filler loading and 0.5 mm particlesize)andsampleB3(80wt%offillerloadingand1mmparticlesize)has maximum MOE. The composites exhibit similar moisture absorption property compared to wood-based particle boards. The sample A1 could be used for the general purpose applications as a particle board. However, it is requiredthatfurtherworktobedoneonfiller–matrixinterphaseoptimizationin order to obtain a composite with the best mechanical performance. The resultsindicatethatthecombinationofsmallerparticlesizeof0.5mmwithhigherwt%offillermaterial (80%) isbeneficial forminimizingthethermalconductivity and thermal expansion of groundnut shell particles reinforced polymercompositematerials.GPECpanelspossessbetterthermalpropertiesand are good substitute for wood based panels in thermal applications such as thermal insulation purpose. The composite board A3 could be used for the general purpose thermal insulation applications. To ascertain their suitability a few more tests are to be carried out to evolve the hydrothermal and weather



resistancepropertiesof thesecomposites. It issuggestedthat theGPECcomposites could be considered as an alternative to wood material in the manufacture of particleboard used in indoor environment due to moderate mechanical properties and relatively lower water absorption. This would be a promising material for structural, packaging, and other general applications. Furthermore, the pressure on other forest resources could be reduced.

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